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ANKYLOSAURS AND REVERSALS



Jon Woolf writes:
 <<You say that the perforate acetabulum is a diagnostic feature for
Dinosauria --
 that is, it's one of the apomorphies that show Dinosauria is a real group,
and also
 one of the apomorphies that can be used to identify a member of that group.
You
 also say that ankylosaurs don't have this apomorphy.  Yet, ankylosaurs are
still
 classified as dinosaurs.  Why?>>

Because ankylosaurs REVERSED this feature.  They went back to the ancestoral
condition of a non-perforate acetabulum.  The same thing happened with "post-
Archaeopteryx" birds.

The reason that Ankylosaurs are classified as dinosaurs, a monophyletic
assemblage consisting of the most recent ancestor of Triceratops and birds and
all of its descendants, is because Ankylosaurs are OBVIOUSLY ornithischians,
which is the Triceratops stem.

The have opisthopubic pelves, predentary bones, and three superorbital
elements, very similar to those of stegosaurs which DO have perforate
acetabula, as well as seven premaxillary teeth ancestorally, like stegosaurs.]

Note however, that Maryanska and Osmolska (or was it Osmolska and Maryanska?)
1984 stated that because ankylosaurs had an imperforate acetabulum, then they
were the most basal ornithischians.

That study was done before wide-spread acceptance of dinosaurian monophyly.
Recent works, especially by Novas, Sereno, and Arcucci (not together) reaffirm
the idea of Bakker and Galton's that a fully perforate acetabulum is
diagnostic of Dinosauria, and that various outgroups such as Marasuchus and
Lagerpeton had varying degrees of acetabular openness.

This is something that must be remembered: EVEN THOUGH SOMETHING IS DIAGNOSTIC
OF A GROUP, IT DOES NOT MEAN THAT IT CANNOT BE TRANSFORMED OR REVERSED.
 
 <<Is or isn't the perforate acetabulum diagnostic for _all_ Dinosauria?  If
it isn't,
 why is it still in the list of diagnostic features?>>

It is diagnostic for Dinosauria as a whole.  Because some dinosaurs reversed
or transformed this character does not alter their status as dinosaurs because
numerous other characters affirm their dinosaurian status: the predentaries,
superorbitals, teeth, pelves etc of ankylosaurs, and the hands, feet, and
faces (among other things) of "post-Archaeopteryx" birds.

Basically what a diagnosis is, is an attempt to figure outt the ancestoral
character states of a taxon and figure out what the ur-creature was like.

 <<If it is, then how can you be sure that ankylosaurs are dinosaurs, when
they don't have
 all the diagnostic features for dinosaurs?>>

Like I said before, they are dinosaurs because they belong on the Triceratops
stem of Dinosauria.  They are ornithischians without a doubt, Ornithischians
are dinosaurs BY DEFINITION, thus, ankylosaurs are dinosaurs.

 <<Are you assuming ankylosaurs are dinosaurs because past
 workers said they were dinosaurs?  Or are there perhaps other features that
are
 more useful than the acetabulum for identifying something as a dinosaur?>>

No, that is not the case at all.  BECAUSE ankylosaurs have an opisthopubic
pelvis, that is very similar to those in other ornithischians, as well as
numerous other ornithischian synapomorphies, yet an imperforate acetabulum,
parsimony dictates that the imperforate acetabulum of ankylosaurs is a
reversal, and not a 'true' plesiomorphic state.
 
 <<To me, saying "X is a dinosaur even though X doesn't have all the
diagnostic
 features of a dinosaur" is as much a contradiction as saying "6 is a prime
 number.">>

That is totally an invalid comparison.  NUMBERS DO NOT GO THROUGH CHARACTER
REVERSALS.  Like I have said two or three times already, animals DO go through
character reversals, and parsimony dictates that because ankylosaurs and
"post-Archaeopteryx" birds do have numerous other synapomorphies that nest
them well within the Dinosauria, they MUST be considered dinosaurs, and that
the imperforate acetabulum must be seen as a reversal.

 <<Your systematics will never make any logical sense as long as you allow
casual
 violations of the laws of logic. >>

If you had any idea of the workings of biology, evolution, and how the
systematics actually work, you would not be so hateful of them.

Peter Buchholz
Tetanurae@aol.com